Adsorptive formed bodies, particularly those having a channel structure and based on activated carbon, are employed preferably in filtering systems used for the removal of hydrocarbons from air streams. In particular, adsorptive formed bodies are being used preferentially in the automotive industry in tank ventilation systems. The channel structure can be obtained by extrusion and offers a large contact area per unit volume. Substantially improved adsorption dynamics or adsorption kinetics are achieved on account of the channel structure, as compared with granulated active carbon or extruded carbon formed bodies.
An activated carbon formed body is disclosed in U.S. Pat. No. 5,488,021. Bentonite, attapulgite, or kaolin can be used as a matrix in which the activated carbon can be embedded. Cellulose ether is added to act as green binder. Since the formed body is dried at a maximum temperature of 100° C. and is not sintered, the cellulose ether is not thermally degraded and restricts the absorption capacity of activated carbon.
The formed body with channel structure which is described in U.S. Pat. No. 5,914,294 is produced using a ceramic sintering process. This is done by mixing activated carbon in powder form with a ceramic-forming material and a flux, and sintering it under a blanket of protective gas after it has been formed and dried. The flux reduces the sintering temperature, but even so, temperatures above 1000° C. must be used in order to ensure adequate rigidity and water insolubility of the formed body.
The drying procedure of the process described in U.S. Pat. No. 5,914,294 is of paramount importance. Owing to the presence of materials in the formed body which dry at different rates, the formed body must be dried slowly, in order to avoid the formation of cracks. For this reason, while drying the formed body, its relative humidity is reduced under controlled conditions, in order to avoid stresses. The drying process is done using vacuum drying or freeze-drying and usually takes about 24 hours.
In U.S. Pat. No. 5,543,096, a silicone resin is used in addition to clay as a binding agent. The formed body is fired at a temperature between 1100° C. and 1300° C., causing amalgamation of clay and SiO2 groups in the silicone resin.
U.S. Pat. No. 5,389,325 and U.S. Pat. No. 5,451,554 describe different methods. U.S. Pat. No. 5,389,325 uses phenolic resin as the binding agent and U.S. Pat. No. 5,451,554 uses epoxy resin as the binding agent. In both cases the resins are crosslinked and cured in a drying step, by means of which rigidity is achieved. However, the temperatures applied are not adequate to free the activated carbon in the formed body from the cellulose ether added and from other thermally destructible additives such that it will exhibit an optimal absorptive capacity.
DE 101 04 882 describes systems in which carbonized resin is used as the binding agent. DE 101 04 882 discloses a very stable formed body, in which the activated carbon particles are embedded in a matrix, and in which the matrix consists of two three-dimensional structures intermingled with each other, made up of carbonized phenolic resin and an inorganic framework that exhibits baked ceramics bonded with the help of a silicate binding agent.
It is an object of the present invention to provide a formed body showing good adsorption characteristics, in which the adsorption agent is securely bonded without restricting its adsorptive capacity to any substantial extent, and which is easy to produce.